The proton-ATPase is a crucial transport enzyme in bacteria, plants and animals. It is also responsible for most of the ATP synthesis in these systems. Our work has shown that the enzyme requires both metal (magnesium) and tightly-bound nucleotide in order to function. Our current research is aimed at finding out how these moieties are used by the enzyme. This is the most complex enzyme known, and its function is of topical importance since it converts chemical and electrical energy at high efficiency at ordinary temperatures.